1. Field of the Invention
The present invention relates generally to an apparatus and method for stabilizing and pasteurizing waste water sludge by the addition of an alkaline material thereto and, specifically, to such a method and apparatus for reacting lime with dewatered waste water sludge for pasteurization, stabilization and disposal thereof.
2. Description of the Prior Art
Raw sludge, such as that obtained from municipal sewage treatment plants, contains large quantities of microorganisms, many of which are fecal in origin. Many of these microorganisms present a potential hazard to humans. As a result, sludge handling makes up a large portion of the capital cost and operating cost of the sewage treatment plant. The increasing scrutiny placed upon municipalities and other waste disposal agencies has increased the need for better stabilization, pasteurization and disposal techniques for waste sludge in order to render such materials either inert, stable, or non-hazardous.
Lime has been used for many years to treat waste materials. For example, lime has been historically used to treat open privies, and the like. In more recent times, studies conducted at municipal waste water treatment plants have shown that lime stabilization can be used to effectively eliminate odors, improve bacterial and pathogenic organism control and to provide a stable byproduct which can be, for example, applied to agricultural land. A number of recent patents discuss the treatment of waste sludge by mixing cement kiln dust, lime or other alkaline materials with the sludge so as to produce a disintegratable product which can be applied to land as a soil conditioner. See, for example, U.S. Pat. Nos. 4,781,842 and 4,554,002 issued to N-Viro Energy Systems, Ltd.
While the above patents show the desirability of mixing cement kiln dust with sludge, they do not specifically disclose the details of an apparatus or method for accomplishing the mixing.
U.S. Pat. No. 4,981,600 to Tobler et al., issued Jan. 1, 1991, shows an apparatus and method for treating waste sludge which comprises mixing cement kiln dust or other alkaline materials with sludge in an elongated auger mixer. An alkaline, additive material is conveyed from an additive hopper to the inlet end of the auger mixer. Sludge is simultaneously introduced to the inlet end of the mixer housing and the auger is rotated and sometimes elevated to mix the additive material and sludge and move the mixed material toward a discharge end of the auger housing.
Although the above apparatus represented an improvement in commercially available sludge mixing technology, the simultaneous introduction of sludge and additive material were found to result in less than even mixing between the alkaline, additive material and the sludge to be treated. The apparatus also required the use of a separate silo for supplying the additive material, as well as additional emissions control equipment. The apparatus was not able to simultaneously mix dirt, sludge and an alkaline additive when required and utilized a short mixer which elevated a single auger.
The present invention has as an object to provide an improved apparatus and method for treating waste sludge by contacting such sludge with an alkaline additive material and ensuring even mixing of the additive material with the sludge.
Another object of the invention is to preserve sanitary landfill capacity by mixing lime along with dirt and waste sludge, the addition of lime serving to lower the volume of dirt required to stabilize the sludge.
Another object of the invention is to provide such an apparatus having a dual compartment supply tank for supplying waste sludge and a stabilizing material such as dirt along with limestone and for varying the relative volumes of stabilizing material and waste sludge entering the apparatus.
Another object of the invention is to provide a mixing apparatus which allows the additive alkaline material to be directly unloaded into an additive hopper without requiring a silo, cyclone separator or other dust control mechanism separate from the mixing apparatus.
Another object of the invention is to provide a mixing apparatus which provides the metered introduction of the additive alkaline material to a mixer housing of the apparatus along a substantial portion of the length thereof in order to ensure more even mixing of the additive material and waste sludge being treated.
Another object of the invention is to provide an apparatus and method for treating waste sludge which is both economical in construction and reliable and efficient in use.
Another object of the invention is to provide an apparatus for treating waste sludge which has a low profile with respect to the ground, whereby sludge can be dumped directly into the apparatus without requiring double handling with a loader.